1. Field of the Invention
The present invention relates to a method of producing a glass body and to an optical glass body produced by this method and to an optical fiber.
2. Description of the Background Art
The modified chemical vapor deposition (MCVD) process is known as a method of producing silica glass to be used as an optical glass body. FIGS. 5A and 5B are conceptual diagrams of the MCVD process. FIG. 5A shows a step for synthesizing glass, and FIG. 5B shows a step for collapsing a glass pipe. In the glass-synthesizing step, a material gas, such as an SiCl4 gas, and an O2 gas are introduced into a glass pipe 1. The glass pipe 1 is heated with a heat source 4 placed at the outside of the glass pipe 1 so as to be movable. The heating causes the material gas to react with the O2 gas. The reaction produces glass particles composed of SiO2 and others. The glass particles are deposited on the inner surface of the glass pipe 1 to form a glass-particle-deposited layer 2. The glass-particle-deposited layer 2 is further heated by the moving heat source 4 and is consolidated to become a synthesized glass layer 3.
In the case of an example shown in FIG. 5A, a GeCl4 gas is also introduced into the glass pipe 1 so that the synthesized-glass layer 3 can be doped with GeO2 to adjust the refractive index. Alternatively, only the glass-particle-deposited layer may be formed in this step by controlling the degree of the heating at the glass-synthesizing step. In this case, the consolidation of the layer is performed in a separate step. The glass pipe 1 having the formed synthesized-glass layer 3 therein is heated by the heat source 4 placed at the outside and is collapsed to become a glass body having a bar shape (see FIG. 5B).
An optical waveguide made with silica glass doped with a rare-earth element such as Er can be used as an amplifier, because when it is shone by an intense incident lightwave of short wavelength, electrons of the ions of the rare-earth element are excited to exhibit an amplifying phenomenon due to stimulated emission. An amplifier incorporating an erbium-doped fiber (EDF) is advantageous in that it is easily connected to a fiber for optical transmission and it has small polarization dependency of the amplification degree. The MCVD process is also used as a method of producing such an optical glass body doped with a rare-earth element.
A method of producing an EDF has been proposed by U.S. Pat. No. 4,826,288 and a literature written by Richard P. Tumminelli et al., (Journal of Lightwave Technology, Vol. 8, pp. 1680–1683). In the proposed method, a glass material gas (such as SiCl4), an Al2Cl6 or AlCl3 gas, and a gas of a chelate compound containing a rare-earth element are transported into a glass pipe with individually separated pipes to be mixed directly before the glass-synthesizing reaction begins. In the method, the glass pipe is heated with a ribbon burner so that the chelate compound will not solidify before reaching the point where the glass-synthesizing reaction is produced by the heating with an oxy-hydrogen burner.
In the above-described method, the chelate compound gas is supplied to the point where the glass-synthesizing reaction occurs. Consequently, much of the water vapor derived from the chelate compound remains in the glass containing the rare-earth oxide (the glass containing the metallic oxide). This causes a problem that the produced fiber has high optical-absorption peaks at 1.24 μm and 1.38 μm due to the vibration of OH groups. The literature written by Richard P. Tumminelli et al. states at the sixth line from the bottom of the left column on page 1682 that the Nd-doped glass has an OH-group content between 15 and 20 ppm.